The present invention generally relates to solid oxide fuel cells and, more particularly, to an improved solid oxide fuel cell stack which allows for crossflow, coflow, counterflow, and radial flow of a fuel and an oxidant.
A fuel cell is basically a galvanic conversion device that electrochemically reacts a fuel with an oxidant within catalytic confines to generate a direct current. A fuel cell typically includes a cathode material which defines a passageway for the oxidant and an anode material which defines a passageway for the fuel. An electrolyte is sandwiched between and separates the cathode and anode materials. An individual electrochemical cell usually generates a relatively small voltage. Thus, to achieve higher voltages that are useful, the individual electrochemical cells are connected together in series to form a stack. Electrical connection between cells is achieved by the use of an electrical interconnect between the cathode and anode of adjacent cells. Also typically included in the stack are ducts or manifolding to conduct the fuel and oxidant into and out of the stack.
The fuel and oxidant fluids are usually gases and are continuously passed through separate cell passageways. Electrochemical conversion occurs at or near the three-phase boundary of the electrodes (cathode and anode) and electrolyte. The fuel is electrochemically reacted with the oxidant to produce a DC electrical output. The anode or fuel electrode enhances the rate at which electrochemical reactions occur on the fuel side. The cathode or oxidant electrode functions similarly on the oxidant side.
Specifically, in a solid oxide fuel cell (SOFC), the fuel reacts with oxide ions on the anode to produce electrons and water, the latter of which is removed in the fuel flow stream. The oxygen reacts with the electrons on the cathode surface to form oxide ions that diffuse through the electrolyte to the anode. The electrons flow from the anode through an external circuit and then to the cathode, with the circuit being closed internally by the transport of oxide ions through the electrolyte.
In a SOFC, the electrolyte is in a solid form. Typically, the electrolyte is made of a nonmetallic ceramic, such as dense yttria-stabilized zirconia (YSZ) ceramic, that is a nonconductor of electrons which ensures that the electrons must pass through the external circuit to do useful work. As such, the electrolyte provides a voltage buildup on opposite sides of the electrolyte, while isolating the fuel and oxidant gases from one another. The anode and cathode are generally porous, with the anode oftentimes being made of nickel/YSZ cermet and the cathode oftentimes being made of doped lanthanum manganite. In the solid oxide fuel cell, hydrogen or a hydrocarbon is commonly used as the fuel, while oxygen or air is used as the oxidant.
Various designs have been employed for an electrical interconnect used in fuel cell stacks. Likewise, different means have been used for constructing fuel/oxidant manifolds or passageways. One interconnect design is found in U.S. Pat. No. 5,460,897 wherein the interconnect assembly not only provides electrical connection between anodes and cathodes, but also provides the means for fuel/oxidant pathways. The interconnect assembly has a manifold plate with two recesses that define transverse flow channels running perpendicular to one another. One channel is used to flow fuel while the other channel is used for the oxidant. An annular shaped bellows is within a central opening that extends perpendicularly through the manifold plate. The bellows accommodates radial dimensional differences between the manifold plate and an interconnect plate. The interconnect plate is disposed within the bellows and has protrusions on both sides for making electrical contact between adjacent cells, as well as providing spacing for flow of the fuel and oxidant between adjacent cells. Disadvantages to such design, however, are its relative complexity in structure, multiple fabrication steps for the interconnect plate, overall thickness of the interconnect plate, and limitation to crossflow of the fuel and oxidant.
In contrast to U.S. Pat. No. 5,460,897 which uses the interconnect to provide fuel/oxidant passageways, U.S. Pat. No. 5,256,499 discloses various shaped anodes and cathodes which provide different fuel/oxidant passageways. A flat interconnect element connects adjacent cells. Some of the shapes for the anodes/cathodes include corrugation, elongated ribs, and rectangular posts. Flat layers of anode and cathode material are added between the electrolyte and shaped anodes and cathodes, respectively, to aid in bonding to the electrolyte and providing surface area for chemical reactions. However, the use of a gasket element which surrounds the fuevoxidant passageways limits the utility to crossflow. Also, having both the anodes and cathodes shaped into something other than a flat configuration tends to increase the overall thickness of the stack and requires multiple fabrication steps.
In a fashion similar to U.S. Pat. No. 5,256,499, corrugated anodes and cathodes with a flat, trilayer electrolyte wall (or interconnect wall) therebetween is shown in U.S. Pat. No. 5,162,167. The trilayer wall includes anode, electrolyte (or interconnect) and cathode materials. The fuel and oxidant flow can be achieved in a coflow or counterflow pattern. And, again, the non-flat configuration of both the anodes and cathodes tends to increase the stack thickness and requires multiple fabrication steps.
An elongated circular configuration for fuel and oxidant passageways formed by anodes and cathodes is shown in U.S. Pat. No. 4,913,982. A flat interconnect is disposed between adjacent cells. Fuel and oxidant flow can be achieved in coflow or counterflow patterns. Another limitation, as with other past designs, is the overall thickness of the stack which is dictated by the shape of both anodes and cathodes, as well as requiring multiple fabrication steps.
As can be seen, there is a need for an improved solid oxide fuel cell stack which is simple in design and reduces the overall thickness of the stack. Also needed is an SOFC stack which allows flexibility in flow of a fuel and an oxidant. In particular, a stack is needed which allows crossflow, coflow, counterflow, and radial flow of the fuel and oxidant without having to alter the stack design for any one particular flow pattern. What is also needed is a stack design which allows both external and internal manifolding to increase flexibility in use of the stack. An additional need is an interconnect which helps achieve the above needs of the solid oxide fuel stack. Yet another need is for an interconnect which requires less material and fewer processing steps in its manufacture.